Military losses have mounted, progress has slowed, and a blame game has begun among some Russian supporters of the war. In January, the head of a group of serving and retired Russian military officers declared that invading Ukraine would be “pointless and extremely dangerous.” It would kill thousands, he said, make Russians and Ukrainians enemies for life, risk a war with NATO and threaten “the existence of Russia itself as a state.” To many Russians, that seemed like a far-fetched scenario, since few imagined that an invasion of Ukraine was really possible. But two months later, as Russia’s advance stalls...